passt, pasta: Run-time selection of AVX2 build

Build-time selection of AVX2 flags and routines is not practical for
distributions, but limiting AVX2 usage to checksum routines with
specific run-time detection doesn't allow for easy performance gains
from auto-vectorisation of batched packet handling routines.

For x86_64, build non-AVX2 and AVX2 binaries, and implement a simple
wrapper replacing the current executable with the AVX2 build if it's
available, and if AVX2 is supported by the current CPU.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

arch.c

@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+/* PASST - Plug A Simple Socket Transport
+ *  for qemu/UNIX domain socket mode
+ *
+ * PASTA - Pack A Subtle Tap Abstraction
+ *  for network namespace/tap device mode
+ *
+ * arch.c - Architecture-specific implementations
+ *
+ * Copyright (c) 2022 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ */
+
+#include <limits.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+/**
+ * arch_avx2_exec() - Run AVX2 build if supported, drop suffix from argv[0]
+ * @argv:	Arguments from command line
+ */
+#ifdef __x86_64__
+void arch_avx2_exec(char **argv)
+{
+	char *p = strstr(argv[0], ".avx2");
+
+	if (p) {
+		*p = 0;
+	} else if (__builtin_cpu_supports("avx2")) {
+		char path[PATH_MAX];
+
+		snprintf(path, PATH_MAX, "%s.avx2", argv[0]);
+		argv[0] = path;
+		execve(path, argv, environ);
+		perror("Can't run AVX2 build, using non-AVX2 version");
+	}
+}
+#else
+void arch_avx2_exec(char **argv) { (void)argv; }
+#endif